Plasma is generally considered to be one of the four states of matter, the others being solid, liquid and gas states. In the plasma state the elementary constituents of a substance are substantially in an ionized form. This form is useful for many applications due to, inter alia, its enhanced reactivity, energy, and suitability for the formation of directed beams.
Plasma generators are routinely used in the manufacture of electronic components, integrated circuits, and medical equipment, and in the operation of a variety of goods and machines. For example, plasma is extensively used (i) to deposit layers of a desired substance, for instance, following a chemical reaction or sputtering from a source, (ii) to etch material with high precision, (iii) to sterilize objects by the free radicals present in the plasma or induced by the plasma, and (iv) to modify surface properties of materials.
Plasma generators based on radio frequency (“RF”) power supplies are often used in experimental and industrial settings since they provide a ready plasma source, and are often portable and easy to relocate. Such plasma generators couple RF radiation to a gas, typically at reduced pressure (and density), causing the gas to ionize. In any RF plasma production system, the plasma represents a variable load at the antenna terminals, which are typically driven by the RF power supply, as the process conditions change. Such variable process conditions include, changes in working gas and pressure, which affect the amount of loading seen at the antenna terminals. In addition, the amplitude of the RF drive waveform itself affects the plasma temperature and density, which in turn also affects the antenna loading. Thus, to the RF power source, the antenna/plasma combination is a non-constant and nonlinear load.
A typical RF source has an output impedance of about 50 ohm, and as a result couples most efficiently to a load that presents a matching 50 ohm impedance. Because of the often unpredictable changes in the plasma self inductance, effective resistance, and mutual inductance to the antenna, provision for dynamic impedance matching is made by retuning some circuit elements and possibly the plasma to obtain satisfactory energy transfer from the RF source to the generated plasma. To achieve this, an adjustable impedance matching network, or “matching box” is typically used to compensate for the variation in load impedance due to changes in plasma conditions.
A typical dynamic matching box contains two independent tunable components: one for adjusting the series impedance and another for adjusting the shunt impedance. These tunable components must be adjusted in tandem with each other in order to achieve the optimum power transfer to the plasma. Not surprisingly, accurate tuning of these components is often a difficult process. Typically, retuning requires both manual/mechanical operations/actuators to adjust one or more component values as the plasma impedance changes and generally sophisticated feedback circuitry for the rather limited degree of automation possible.
It is well known that the application of a sufficiently large electric field to a gas separates electrons from the positively charged nuclei within the gas atoms, thus ionizing the gas and forming the electrically conductive fluid-like substance known as plasma. Coupling radio frequency electric and magnetic fields to the gas, via an antenna, induces currents within this ionized gas. This, in turn, causes the gas to further ionize, thereby increasing its electrical conductivity, which then increases the efficiency with which the antenna fields couple to the charged particles within the gas. This leads to a further increase in the induced currents, resulting in the progressive electrical breakdown and substantial ionization of the gas. The effectiveness of the RF coupling is dependent upon the particular RF fields and/or waves that are used. Some RF field configurations and waves that are suitable for the efficient production of large volumes of plasma are described next.
Whistler waves are right-hand-circularly-polarized electromagnetic waves (sometimes referred to as R-waves) that can propagate in an infinite plasma immersed in a static magnetic field Bo. If these waves are generated in a finite plasma, such as a cylinder, the existence of boundary conditions—i.e. the fact that the system is not infinite—cause a left-hand-circularly-polarized mode (L-wave) to exist simultaneously, together with an electrostatic contribution to the total wave field. These “bounded Whistler” are known as Helicon waves. See Boswell, R. W., Plasma Phys. 26,1147 (1981). Their interesting and useful qualities include: (1) production and sustenance of a relatively high-density plasma with an efficiency greater than that of other RF plasma production techniques, (2) plasma densities of up to Np˜1014 particles per cubic centimeter in relatively small devices with only a few kW of RF input power, (3) stable and relatively quiescent plasmas in most cases, (4) high degree of plasma uniformity, and (5) plasma production over a wide pressure range, from a fraction of a mTorr to many tens of mTorr. Significant plasma enhancement associated with helicon mode excitation is observed at relatively low Bo-fields, which are easily and economically produced using inexpensive components.
Significant plasma density (Np) enhancement and uniformity may be achieved by excitation of a low-field m=+1 helicon R-wave in a relatively compact chamber with Bo<150 G. This may be achieved, for instance, through the use of an antenna whose field pattern resembles, and thus couples to, one or more helicon modes that occupy the same volume as the antenna field. The appropriate set of combined conditions include the applied magnetic field Bo, RF frequency (FRF),), the density Np itself, and physical dimensions.
Some antenna designs for coupling RF power to a plasma are disclosed by U.S. Pat. Nos. 4,792,732, 6,264,812 and 6,304,036. However, these designs are relatively complex often requiring custom components that increase the cost of system acquisition and maintenance. Moreover, not all of the designs are suitable for efficient generation of the helicon mode, which is a preferred mode disclosed herein.
RF power sources typically receive an external RF signal as input or include an RF signal generating circuit. In many processing applications, this RF signal is at a frequency of about 13.56 MHz although this invention is not limited to operation at this frequency. The RF signal is amplified by a power output stage and then coupled via an antenna to a gas/plasma in a plasma generator for the production of plasma.
Amplifiers, including RF amplifiers suitable for RF power sources, are conventionally divided into various classes based on their performance characteristics such as efficiency, linearity, amplification, impedance, and the like, and intended applications. In power amplification, an important concern is the amount of power wasted as heat, since heat sinks must be provided to dissipate the heat and, in turn, increase the size of devices using an inefficient amplifier. A characteristic of interest is the output impedance presented by an amplifier since it sets inherent limitations on the power wasted by an amplifier.
Typical RF amplifiers are designed to present a standard output impedance of 50 Ohms. Since, the voltage across and current through the output terminals of such an amplifier are both non-zero, their product provides an estimate of the power dissipated by the amplifier.
This product can be reduced by introducing a phase difference between the voltage and the current across the output terminals of the amplifier in analogy with the power dissipated in a switch. In contrast to conventional amplifiers, a switch presents two states: it is either ON, corresponding to a short circuit, i.e., low impedance, or OFF, corresponding to an open circuit, i.e., infinite (or at least a vary large) impedance. In switched mode amplifiers, the amplifier element acts as a switch under the control of the signal to be amplified. By suitably shaping the signals, for instance with a matching load network, it is possible to introduce a phase difference between the current and the voltage such that they are out of phase to minimize the power dissipation in the switch element. In other words, if the current is high, the voltage is low or even zero and vice versa. U.S. Pat. Nos. 3,919,656 and 5,187,580 disclose various voltage/current relationships for reducing or even minimizing the power dissipated in a switched mode amplifier.
U.S. Pat. No. 5,747,935 discloses switched mode RF amplifiers and matching load networks in which the impedance presented at the desired frequency is high while harmonics of the fundamental are short circuited to better stabilize the RF power source in view of plasma impedance variations. These matching networks add to the complexity for operation with a switched mode power supply rather than eliminate the dynamic matching network. Such a matching load network is also not very frequency agile since it depends on strong selection for a narrow frequency band about the fundamental.
U.S. Pat. No. 6,432,260 discloses use of switched elements in matching impedance networks to ensure that the dynamic complex impedance of the plasma is seen as a near resistive value, effectively neutralizing the reactive components of plasma impedance. This allows a power source to only respond to resistive changes in the plasma since it is only such changes that are seen by the power source. The dynamic plasma resistance controls the power delivered to the plasma.
When plasma impedance is a small fraction of the impedance seen by the RF source, variations in plasma impedance are a relatively less significant. Thus, it is possible to drive a plasma with an RF power supply without an intervening dynamic matching network if the range of plasma impedance variations is a small fraction of the total impedance seen by the source. Overwhelming the plasma inductance with a sufficiently high power driver results in compromising efficiency to some extent. As a result, a matching network is required when the dynamic plasma impedance is a significant fraction of the total impedance seen by the RF power source.
U.S. Pat. Nos. 6,150,628, 6,388,226, 6,486,431, and 6,552,296 disclose constant current switching mode RF power supply containing an inductive element in series with the plasma load. The plasma is primarily driven as the secondary of a iron- or ferrite-core transformer, the primary of which is driven by the RF power supply. In such a configuration, a dynamic impedance matching network is disclosed to be not required. The current through the plasma is maintained at about the value of the initial inductor current to adjust the power based on the size of the load.
The above patents also disclose various methods for igniting a plasma that include high voltage pulses, ultra-violet light and capacitative coupling, which also serve to restrict variations in plasma impedance by sidestepping the large impedance variations typically encountered upon plasma ignition.
There are other known designs that use the plasma as a secondary in a transformer-like design in which the secondary and the primary are relatively weakly coupled via a shared core. R. J. Taylor developed a plasma production technique for cleaning the inside of a toroidal vacuum chamber using a process plasma, and had built such a device in 1973. The circuit used the air-core Ohmic Heating (OH) winding of a tokamak as its transformer primary, and a matching network consisting of fixed C1 and C2. Similar designs operating on other tokamaks, some having iron-core transformers, are known. These designs typically operate in the frequency range 1–50 kHz.
In designs similar to those of R. J. Taylor, the changes in the plasma impedance do not significantly affect the loading of the driver because the parameter
      δ    ≡                  M                  1          ⁢          p                                                  L            1                    ⁢                      L            p                                ,where M1p is the mutual inductance between the primary inductance L1 and the plasma inductance Lp, is quite small. Consequently, variation in the inductive load seen at the terminals of the transformer primary is smaller. In contrast, when the plasma is substantially directly driven, e.g., via current-straps, where
      δ    ≡                  M                  ant          -          plasma                                                  L            ant                    ⁢                      L            plasma                                ,wherein Mant-plasma is the mutual inductance between the antenna inductance Lant and the plasma inductance Lplasma, is not small, and as a result changes in the plasma impedance represent relatively larger changes in the load impedance seen by the RF source. This variation typically requires the use of variable matching network to provide a reasonable match with a 50 Ohm impedance of the RF source for delivering power.
When plasma is driven directly, i.e., without a core for substantially coupling a plasma secondary to a primary winding connected to the RF source, changes in plasma impedance are significant at the leads of the antenna or at the primary winding of a coupling transformer. This configuration has been coupled to a plasma or plasma/antenna combination via a dynamic matching network that may be continually adjusted in response to the changing plasma impedance.
The problems faced in an efficient plasma generator design include the need for a low maintenance and easily configured antenna, the elimination of expensive dynamic matching networks for directly coupling the RF power source to the non-linear dynamic impedance presented by a plasma, and the need for RF power sources that can be efficiently modulated and are frequency agile.